1. Field Of The Invention
The invention relates generally to voltage sensing circuits for power control and power management in applications such as intelligent power supplies and industrial automation systems, and, more specifically, to a circuit for sensing voltages beyond the voltage limits of the power supplies which operate the components in the circuit.
2. Description Of The Relevant Art
Reduced to bare essentials, an industrial process may be regarded as having a number of sensors and loads that correspond to input and output variables for a process control computer system. The sensors provide input values representative of the state of the process at a given time: the loads respond to output values, and thereby control various aspects of the process. Typical sensors include relay contacts, proximity switches, and pressure switches. Typical loads include contractor coils of starters for large motors, solenoid valves, relays, lamps, and small motors. A process may have several hundred to several thousand input sensors and loads that must be serviced at very frequent intervals.
A typical computer system for automating an industrial process contains a number of general and special purpose computers. The system monitors input variables from the process, performs suitable logical manipulations on the inputs, and updates output variables for the process. The computer system is usually organized hierarchically. A host processor, typically a minicomputer or a mainframe, communicates with a number of programmable controllers, each of which communicates with a number of power control subsystems through a number of local processors. A programmable controller is a processor designed specifically to perform logical manipulations on a large number of binary inputs on a cyclical basis. The local processors have as their primary function the efficient transfer of data between the power control subsystems and the working memories of the programmable controllers. The power control subsystems provide the interface between the local processors and the various sensors and loads.
In order to provide operational information to the system monitor load parameters must be sensed and communicated to the host processor. The load parameters typically comprise voltages which exist at various load points throughout the system. A common test point is a power switch (FET or IGBT) which switches power to the load. One method of sensing voltages employs the use of operational amplifiers with suitable feedback elements. Op amps typically operate from power supplies which generate approximately 5 to 12 volts and, for each parameter that is to be tested, require 4 resistors which must be ratio-matched in pairs: one for the feedback loop, one for the reference voltage, and one for each of the two input terminals of the op amp for scaling the voltages from the sensors to the reference voltages set by the reference resistor. The latter are required because operational amplifiers cannot accept signals beyond their supply voltage.
In op amp-based sensing circuits, dynamic range is limited because of the requirement of scaling the system to voltages and the inherent error of the resistors. Additionally, each resistor typically must be placed externally, and for every new parameter that is to be tested, four new resistors are required. Furthermore, when voltages at the power switch are being monitored, the setup of the operational amplifier depends on whether the op amp is placed on the high or low side of the line. Op amp configurations for high-side op amp grounding are not easily interchangeable with op amp configurations for low-side op amp grounding. Finally, separate operational amplifiers ordinarily must be provided for AC and DC signals. Consequently, systems which employ operational amplifiers to sense voltages are severely limited in flexibility.